Single-photon avalanche photodiodes 100 (SPADs) (FIG. 1) typically are P-I-N diodes that operate at high bias; such diodes have a P-doped (P) region 104 and an N-doped (N) region 106 formed adjacent to each other, or with a thin intrinsic (I) region lying between them, in a semiconductor material. The P region 104 and N region 106 are sufficiently close to each other that, with an applied reverse voltage bias, majority carriers are swept from a depletion zone 102 formed in a region between the N and P regions; with majority carriers swept from the depletion zone little or no current flows between the N and P regions.
Photons are admitted into the photodiode through a front or back surface into the photodiode. Absorption of photons in the photodiode, whether in depletion zone 102 or in the N 206 or P 204 regions adjacent the depletion zone, causes, through the photoelectric effect, release of at least one electron-hole carrier pair that is attracted into the depletion zone 102 of the photodiode. The N or P regions may have subregions, not shown, of differing doping concentration to enhance photon absorption. Applied voltage bias is high enough that carriers such as a photoelectric electron-hole pair are amplified as the carriers trigger release of more electron-hole pairs in an avalanche breakdown, and gives a surge of current between the P 104 and N 106 regions. Interconnect 108 couples to the N region 106, and interconnect 110 to the P region 104 to connect the photodiode to other circuitry such as bias circuitry and sensing circuitry, not shown; one of N 104 and P 106 regions may be common to multiple photodiodes.
The avalanche photodiode is biased with circuitry configured so this surge of current typically drops bias voltage enough to “quench,” or stop, current flow, or once current flow is detected the bias circuitry removes bias to quench current flow, after current flow is quenched bias is reset for further photon detection. Each surge of current triggered by each absorbed photon generates a signal that is amplified as an electronic indication of photon absorption.
SPADs may avalanche without photon absorption of desired light, giving extra, undesired, current surges, known as the dark count, that may be erroneously interpreted as indicating photon absorption. Since SPADs may respond to high-energy photons such as cosmic-ray, gamma and X-ray radiation, a portion of the dark count represents responses to undesired radiation. A portion of the dark count also results when minority carriers in the P 104 and N 106 regions are drawn into and amplified within the depletion zone 102.
It is known that some minority carriers are introduced at flaws in oxide-silicon interfaces. The interface between N region 106 and overlying dielectric oxide 114 is an example of such an interface.
The photodiode 100 typically has a frontside 115 into which diffusion and implant steps are performed during fabrication, and upon which interconnect metallization, such as metal 108, 110 are formed within dielectric oxide 114. The photodiode typically also has a backside 117 lacking metal interconnect lines 108, 110, although metal shield and mounting layers may be present. Backside 117 typically begins as a silicon or other semiconductor substrate upon which active layers of the photodiode are formed. In backside-illuminated devices, much of the semiconductor substrate is removed and typically a protective and stabilizing backside oxide or other transparent passivation coating is deposited after removal of the substrate. Both frontside and backside-illuminated photosensor arrays therefore overlay a layer 116 that includes an oxide, a substrate, or a non-oxide transparent passivation coating.